In extractive metallurgy autoclaves are used for allowing increased operating temperature in the autoclave. Once an ore or concentrate is leached the discharge of the autoclave is often reduced in temperature and pressure by allowing the autoclave discharge slurry to flash i.e. convert the heat of the slurry at high temperature into a flash steam. To reduce capital cost in base metal autoclave operations the number of flash stages is minimised. However the pressure drop per flash stage is commonly high and the flashing slurry discharges into the flash tank at high velocity. Typically top-entry flash vessel design is used allowing the high pressure and high velocity flashing slurry to discharge into a slurry pool to absorb the kinetic energy of the slurry. This avoids excessive wear in the flash vessel that would occur if a bottom-entry design (used in alumina industry) were to be used.
The slurry flowing from one flash vessel to another flash vessel is at the boiling point of the slurry leaving the upstream vessel and if a pressure drop occurs in the pipeline interconnecting the two flash vessels then the slurry will boil and as steam is generated three-phase flow will occur. Since the volume of the steam is much greater that the equivalent volume of the liquid water, the velocity of the three-phase mixture increases substantially. This increased velocity will lead to excessive pipeline and valve wear.
Typically top-entry multi stage flash vessels are arranged staggered in height to ensure that three-phase flow is avoided in the pipeline connecting the two flash vessels to ensure no loss in pipeline pressure due to a change in static height. However elevating vessels is very expensive as the flash vessels are large and heavy pressure vessels that are brick lined vessels requiring substantial structural steel support and cost for their installation.
However, if top-entry flash vessels are constructed at the same level then a static height difference between the vessels will occur. This will lead to a pressure drop in the interconnecting pipeline and the slurry will boil leading to three phase flow at very high velocities and excessive wear.
To keep the flash vessels all at the same level and to avoid boiling in the pipeline between the flash vessels quench water is commonly used to sub-cool the slurry entering the pipeline in between the flash vessels.
Adding water injection facilities to a flash vessel is expensive and the operation of the circuit becomes more complicated in terms of operability. Furthermore, additional water is required, which may further impact the water balance of the system.